At present, protective coatings for polycarbonate available commercially are acrylic lacquers, organosilicones or polysiloxanes applied by powder technique or immersion, if desired dried by ultraviolet. It has been proposed to replace these coatings by amorphic alloys of silicon deposited at ambient temperature by PECVD ("plasma enhanced chemical vapor deposition") from silane. Such a process is for example described in French patents 2,614,317 and 2,631,346. One of the major problems to be solved is to ensure good resistance of the coating to large variations of temperature, given the differences of coefficients of expansion of the layer and of the substrate, even though these products are subject to strict specifications as to behavior as to cycles and thermal shock.
In French application No. 90.05529 of May 2, 1990 in the name of the applicant, an anti-UV coating of the absorptive type has been proposed, using an amorphous alloy of silicone deposited by PECVD from silane, of which the cracking threshold is adjusted via the composition by choosing appropriate values of the different gas flows. If an improvement of the behavior under ultraviolet radiation (UV) is effectively observed for moderate radiation densities (laboratory UV lamp), the coatings effected generally do not pass the accelerated tests under very strong intensity recommended by standardization groups.
This behavior can be explained if one considers the variation of the coefficient of absorption in semiconductors and amorphous insulators. At low energy, the absorption results from transitions between localized conditions of the intercepted band. It is much less intense and the corresponding spectral region is considered as transparent. At medium energy, the absorption results from transitions between localized states at the edge of the band of valence and of conduction, which result from the partially disordered structure of the material, and extended conditions of the opposite band. The experimental form of the coefficient of absorption in this zone is called "Urbach's tail" and is well described by the relationship: EQU .alpha.=.alpha..sub.0 exp(h.nu.-E.sub.1)/E.sub.0)
in which .alpha..sub.0 and E.sub.1 are independent of disorder. E.sub.0 measured 1' "extension" of the Urbach tail and constitutes an indirect measure of the structural disorder.
Finally, at high energy, the absorption results from electronic transitions between widespread states from band to band. It is currently described by the Tauc's law: EQU (ah.nu.).sup.1/2 =B(H-E.sub.G).
The parameter B is representative of the "speed of variation" of .alpha. at high energy, while E.sub.G defines the threshold energy of the photon, above which the material can be considered as totally opaque. By analogy with crystalline solids, E.sub.G is called width of absorbed optical band (optical "gap").
To produce an absorptive UV filter, there is deposited an amorphous alloy of silicon whose optical "gap" will be adjacent the limit between the visible and the ultraviolet. The ideal filter should have a profile that is in the form of a stair step so as to stop all UV radiation while remaining completely transparent in the visible range. This is far from being the case in practice.
If one considers the coefficient of experimental transmission of a layer of an alloy a-SiN.sub.y :H of which the "gap" is about 350 nm, of about 1 .mu.m thickness, as a function of the wavelength, there will be noted three zones characteristic of the different kinds of absorption, mentioned previously: quasi-transparent, Urbach tail, strong "parabolic" absorption. The transition between semi-transparency and substantially no transmission has a band of wavelength which is far from being negligible. If this region is centered on the limit UV/visible (about 350 nm), certain wavelengths of the visible (violet-blue) are particularly attenuated, giving rise to the yellowish coloration of the specimen, although a portion of the UV spectrum is transmitted, particularly around 365 nm, a wavelength very damaging to polycarbonate. If one uses a material of smaller "gap", so as to render the transmission substantially zero over all the UV range, the film will appear dark yellow orange or even brownish (color characteristic of amorphous silicon). Few applications are known in which this characteristic would be acceptable. One cannot on the other hand hope to act on the optical properties of the material to reduce the width of the transition between weak and strong absorption, which is to say to increase the multiplier B of the. Tauc's law and reduce the parameter E.sub.0. Thus, it appears that B and E.sub.0 will be, in the first instance, functions uniquely of the relative incorporation of oxygen, nitrogen, hydrogen relative to silicon, which is dictated by the choice of the value of the optical "gap".
Another theoretical solution would be to reinforce the slope of the gap by increasing the thickness of the deposited absorbent material, the transmission varying in the range of strong absorption as .alpha. (n,k) e.sup.-.alpha.d in which .alpha. is a rational function. The simulation shows that the necessary thicknesses will be several microns, which is entirely unrealistic both technologically and economically. Thus, the thermomechanical specifications are ever more difficult to achieve, the greater the thickness of the layers. On the other hand, the absorbent layer is formed from a sub-stoichiometric material of medium hardness and should be associated with a distinctly hard structural layer, also of several microns. The cumulative thickness of material, and hence the time to produce the film, will thus be greatly increased.
Until now, the anti-UV function for a polycarbonate has been produced only for coatings deposited from liquid. An appropriate absorbent molecule is included in the product to be applied. It can be a matter, according to the case, of only the primary adherence (thickness about 1 .mu.m) or of the assembly of the coating (thickness about 5 .mu.m). It will be seen that the results of tests are unsatisfactory except in this last case.
The coatings deposited by PECVD generally require a first liquid adherence which also ensures the anti-UV effect, with the restrictions that have been mentioned. There is no known analog, in PECVD, to the molecules absorptive of UV employed in the case of varnish. The requirement to add in some way a layer applied from a liquid to the PECVD process would sink the project economically.
Another very important factor to be taken into account is the thermomechanical behavior of the anti-UV filter itself. In French patent application 90.05529 of May 2, 1990, there is described the production of absorptive UV filters from oxides, oxynitrides and nitrides of silicon which is hydrogenated but slightly carbonated. For certain applications, very severe accelerated tests are imposed, by exposure to a Xenon arc lamp (.apprxeq.0.1 W/cm.sup.2) for 2,000 hours. However, there has been noted a certain very rapid degradation of the oxide and oxynitride materials during this test, with opening of the optical "gap" and loss of the selective absorption properties.
Only the material called "nitride" which is to say deposited solely from silane and nitrogen, is not deteriorated by the accelerated course.
However, this material has among other things a very serious drawback, namely its mediocre thermomechanical behavior. If one produces the following structure according to the process described in FR-2,631,346, namely: activation of the polycarbonate, adhesive layer of Si:H, anti-UV layer SiN.sub.Y :H, hard layer SiO.sub.2 :H, the specimen obtained will not resist conventional thermal cycle tests.
Given the good thermomechanical behavior of layers of silica, there have been produced specimens in which a layer of SiO.sub.2 :H is inserted between the adhesive layer and the anti-UV layer SiN.sub.y :H. Experience shows that a significant improvement of the thermomechanical properties is obtained only with a prohibitively thick intermediate layer of silica. The performance is nevertheless still insufficient with respect to the specific certain users.